Evap Question

[LPorc]

I've seen a lot on the web about evap cooling of air, and evap cooling of liquid, and they all seem to be saying the same thing.

I know a bong tower will cool the water as the energetic water molecules evaporate, leaving less energy in the water.

I know a swamp cooler will blow air cooled to at best the wet bulb temperature.

The same mechanism of evaporation is used in both cases, are both the air and water cooled in both cases? In the swamp cooler, is the water is being cooled by the evaporation, and then the greater specific heat of the liquid water compared to the combination of air and water vapor results in a entropy temperature closer to that of the liquid water?

I am just trying to wrap my mind around what's happenning, and unfortunately I have no access to a swamp cooler so I can't just let it run and then check the water temperature, and I don't have a bong tower to check the exit air temperature to verify my hypothesis, and all the links I've been through so far for evaporative cooling haven't been much help.

Anyone got any good links or insights?

[LPorc]

chirp chirp...

Silence. Not even a harsh response detailing my ignorance :shrug: . Wow, things have changed around here!

I know I'm ignorant and lazy and just not Googling the right stuff. Any hints are welcome.

[Long Haired Git]

No links. But the energy that was heat of the liquid water and heat of the air has gone into evaporation (phase change) of the liquid. The latent-heat-of-evaporation is massive compared to the energy changing water's temperature.
So, I can see where both the air and liquid reduce their temperature due to the energy removed from the liquid to evaporate some liquid.
So, it looks like everyone wins with evaporative cooling, except of course the extra water in the air has a habit of falling out, and you have to keep sticking more liquid back in.

[LPorc]

Originally Posted by Long Haired Git

No links. But the energy that was heat of the liquid water and heat of the air has gone into evaporation (phase change) of the liquid. The latent-heat-of-evaporation is massive compared to the energy changing water's temperature.

Ah, thank you! Latent Heat of Evaporation did the trick for me on Google. I vaguely remember it from middle school 30 or so years ago as it related to distilling. You know, heat the mixture to the boiling point of the component you are trying to extract, and as long as the temperature holds at that point capture the vapors. The temperature holds steady because the applied heat is going to the phase change.

Apparently these days the vogue term is Specific Latent Heat of Vaporization. There were also some interesting tidbits I followed from the commercial HVAC perspective, and USA Today of all places had an interesting article about weather.

The bottom line is the heat energy to change water state from liquid to gas is very high compared to the amount required to change the temperature of liquid water, so a little evaporation can result in a lot of cooling.

I'm old, I like calories - the amount of energy required to raise the temperature of one gram of water one degree Centigrade (oh yeah, I guess we call that Celsius these days). It takes about 600 calories per gram for the state change from liquid water to water vapor. So, to make things easy, if you start with 1001 grams of water (1.001 liters) and evaporate off 1 gram, the latent heat of vaporization is enough to cool the remaining water .6 degrees C.

So it is the water that is cooled by the evaporation and that the cooling of the air is due to heat transfer from the air to the water, and that the contrast of the specific heats involved, including the heat required for the state change, make it work out that both the air and the water are cooled. Cool.

Thanks for the prod to get me past my mental block! I am smart enough, but never made it past high school (where I paid scant attention to this stuff) and it's hard to remember back that far.

[Ice Czar]

here on the high desert plateau you'll get a good kick out of evaporative cooling
its well worth doing even as a precooler for a phase change condenser

"bong coolers" where never very effcient designs simply easy to make
(forced draft generally without a fill)
you do far better with traditional swap cooler or industrial evaporative chiller designs
adapted to your scale
http://www.google.com/url?sa=t&ct=re...e93SisWSCPMp8g
http://www.cti.org/cgi-bin/download.pl
http://www.airah.org.au/tec_inf.asp

decent modern packing\fill would drastically increase the interface